Vehicle powertrains can include an internal combustion engine connected to an automatic transmission via a coupling device, for example, a torque converter to transfer engine torque to the transmission. The torque converter provides a fluid coupling that transfers torque between an output shaft of the engine and an input shaft of the transmission. The fluid coupling creates a torque converter slip when an impeller of the torque converter connected with the output shaft rotates at a rate that is different than a turbine of the torque converter connected with the input shaft. In some examples, torque converters are designed to slip at low vehicle speeds to enable the transmission to rotate at a slower rate relative to the engine. As the vehicle speed increases and/or other threshold vehicle operating conditions are met, the slip rate gradually diminishes. However, fuel efficiency may be reduced and/or only a portion of available engine power utilized as a result of slip.
In some examples, torque converters can include a lockup clutch that joins the separate rotating turbine and impeller in a direct mechanical connection to minimize slip and improve transmission efficiency of the torque converter. Engagement of the clutch can be determined and applied in relation to threshold vehicle operating conditions. In some examples, control systems of a vehicle attempt to execute a target slip for the torque converter according to vehicle operating conditions and control an engagement force of the clutch until the clutch is fully engaged or locked up. This disclosure describes an improvement over these prior art technologies.